SALT LAKE CITY (AP) — Once a year, hundreds of horseback riders gather on an island in Utah’s Great Salt Lake to nudge bison toward holding pens in a roundup of one of America’s largest and oldest public herds.

Bison have occupied Antelope Island, about 25 miles (80 kilometers) northwest of Salt Lake City, for more than 120 years, after a dozen of them were brought there by a homesteader. Today, several hundred bison roam the island, which is now a state park.

The animals are rounded up each fall so they can receive health checkups and vaccinations and be affixed with a small external computer chip that stores health information.

They are then released back on the island or sold at a public auction to keep the herd at a manageable level of about 500. The island has no natural predators to keep the herd from growing, and there’s a limited amount of grass for the bison to eat, park ranger Charity Owens said.

This year, about 700 bison were pushed into corrals during the 32nd year of a roundup that conjures memories of a bygone era of the American West.

Some of the horseback riders wore old Western clothing, and many donned wide-brimmed cowboy hats. Yells of “Yip ... h’yah” echoed across the island as the riders worked to keep the bison marching through wild grasses on hilly terrain.

Anybody can volunteer to be ride horseback during the roundup, but officials cap it at 250 per year, normally reaching that limit.

Rider Jess Reid, 69, a real estate broker from Park City, said the excitement of the event keeps him coming back.

“Running your horse full speed with stampeding buffalo,” Reid said. “I just don’t know where else in the world, hardly, you could do that?”